A 67-year-old man with hairy-cell leukemia presented to the clinic with a 3-day history of fevers (with temperatures as high as 38.7°C), night sweats, arthralgias, and an erythematous, vesicular-appearing rash on his back. He had had no sick contacts, had not traveled outside the United States for more than 20 years, and had no pets at home. He reported no headaches, shortness of breath, cough, rhinorrhea, bleeding episodes, nausea, vomiting, or diarrhea.Infectious complications in patients with hairy-cell leukemia are common and would be my initial source of concern. Given the nature of the rash, I would also suspect disseminated viral processes, such as a herpes or varicella infection. Other viruses, such as coxsackievirus and enterovirus, may also cause a similar exanthem. Drug eruptions, early Stevens-Johnson syndrome, contact dermatitis, and autoimmune blistering diseases may also be manifested in this manner.The diagnosis of hairy-cell leukemia was made 17 years before the patient's current presentation. At that time, he had a response to chemotherapy with cladribine. The patient had a relapse 10 years before presentation, at which time the administration of cladribine was again effective, although in this instance treatment was complicated by a cardiomyopathy that subsequently resolved. Five months before his current presentation, he received eight doses of rituximab for pancytopenia due to recurrent disease, but his blood counts did not improve.The patient's medical history was otherwise notable for prostate cancer, for which he had undergone a curative radical prostatectomy 12 years ago, and hyperlipidemia. There had been no recent change in his medications, which included 10 mg of simvastatin nightly and 81 mg of aspirin daily. He also took a multivitamin. The patient was born in the United States, was retired from work in the television industry, and was married, with two children. He did not smoke, drink alcohol, or use illicit drugs.On physical examination, he appeared well and was in no acute distress. His temperature was 38.3°C, heart rate 82 beats per minute, blood pressure 136/80 mm Hg, respiratory rate 16 breaths per minute, and oxygen saturation 99% while breathing ambient air. The mucous membranes were moist. The chest examination was normal; the cardiac examination revealed a normal rate and rhythm, with no murmurs.In this Journal feature, information about a real patient is presented in stages (boldface type) to an expert clinician, who responds to the information, sharing his or her reasoning with the reader (regular type). The authors' commentary follows. The New England Journal of Medicine Downloaded from nejm.org at UNIVERSITY OF OTAGO on July 3, 2015. For personal use only. No other uses without permission.